Mulder had green urine when he was ill with pneumonia - the first time I
noticed it was the day after I took him to the vet and he was on Betamox.
I thought it was from the antibiotics.
 
The next time the vet & I noticed it was on the Thursday afternoon and she
thought it was from the vitamins he was having.
 
The next day he was dead.
 
After that the vet thought it was because the infection had got into his
bloodstream and damaged his kidneys.
 
I'm not suggesting that green urine means your ferret is very sick (Mulder
was really ill at the time) but it might mean something's wrong.  Unless
you can narrow it down to something he ingested, I'd take him to the vet
for a check-up.
 
[Posted in FML issue 2983]